Hiya all.

I had 5 discus, all happy, healthy,  and active. Then suddenly one went completely psycho racing around the tank and smacking into everything, then dropped dead. A few days later a second one just dropped dead - no psycho behavior beforehand. Neither one had any visible signs of illness, no parasites, no fungus, but a few hours before dying the second one lost colour and went listless.  All the other fish in the tank were fine, including the other discus. The tank parameters were all fine, nothing new had been added to the tank, no change in feeding, water change had been several days earlier, so no sudden changes in the tank. 

I waited two weeks, to see if any more died, but none did, so I bought another two to replace the dead ones. After adding the two new discus, the next day one of the other discus promptly dropped dead. Again, no signs of illness except loss of colour and listlessness a few hours before dying. As before, all the other fish, including the other discus, are fine. But then so we're the others before they dropped dead.

Any ideas? 

Edited June 29, 201610 yr by zombieworm

Are you using a chlorine remover?

Always, and as I said, there had been no water change when the deaths occurred. 

Rules that out then

 

Is that a botia in there?

if it is, they're bad medicine at night for your discus.